The invention described herein was made in the course of work under Contract No. F33615-76-C-5109 with the United States Government.
Considerable research efforts in recent years have been directed toward the synthesis, characterization, and fabrication of extended chain or rod-like polymers. The unique ordering properties of these polymers into liquid crystalline solutions has led to the preparation of extremely high modulus/high strength fibers. The aromatic polyamides and the aromatic polyamide-hydrazides are members of this class of rod-like polymers. They owe their liquid crystalline behavior to the linearity of monomer catenation and the predominately trans-configuration of the amide group, which is formed in the polycondensation reaction.
However, it has been observed that the thermo-oxidative and hydrolytic stability of amide linkage-containing polymers does not meet some advanced aerospace requirements. It has been concluded that the requisite stability can only be achieved through the use of totally aromatic polymers, preferably those prepared by condensation reactions which form aromatic or heterocyclic fused ring systems which have 180.degree. catenation of the repeat unit.
Earlier work with polymers of this character concerned itself with the preparation and testing of a compound referred to as "PBO". This polymer, poly {[benzo(1,2-d:5,4-d')bisoxazole-2,6-diyl]-1,4-phenylene} was synthesized by the condensation of 4,6-diaminoresorcinol dihydrochloride with terephthalic acid in polyphosphoric acid, and has the formula ##STR2## PBO is described in "Potential Approach to Non-reinforced Composites" by T. E. Helminiak, F. E. Arnold, and C. L. Benner, Polymer Preprints, 16, (2), pp 659-662 (1975). While PBO exhibited improved thermo-oxidative stability and formed liquid crystalline solutions in methanesulfonic acid (MSA) and chlorosulfonic acid (CSA), the highest intrinsic viscosity obtained for it was 3.7 dl/g in MSA at 30.degree. C. For a rod-like polymer of this nature this value represents a number average molecular weight of less than 10,000.
An object of the present invention is to provide a structural analog of PBO having a greatly increased molecular weight as well as improved thermal stability, as compared thereto.